Renewable and Efficient
`Electric Power Systems
`
`_ Gilbert M. Masters
`
`Stanford University ,
`
`
`
`WILEY-
`.INTERSCIENCE '
`.
`.
`JOHN WILEY & SONS, INC, PUBLICATION
`
`*
`
`GE2013
`Vestas v. GE
`|PR2018-01015
`
`GE 2013
`Vestas v. GE
`IPR2018-01015
`
`i
`
`

`

`.‘\
`‘2J
`
`Copyright © 2004 Ey John Wiley & Sons, Inc., Hoboken, New Jersey. All rights reserved.
`
`Published by John Wiley & Sons, Inc., Hoboken, New Jersey.
`Published simultaneously in Canada.
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`Library of Congress Cataloging—in-Publicatian Data
`
`Masters, Gilbert M.
`
`Renewable and efficient electric power systems / Gilbert M. Masters.
`p. cm.
`
`Includes bibliographical references and index.
`ISBN 0—471—28060-7 (cloth)
`,
`1. Electric power systems—Energy conservation. 2. Electric power systems—«Electric
`losses. 1. Title
`
`TK1005.M33 2004
`
`621 .3 1—dc22
`
`2003062035
`
`Printed in the United States of America.
`
`10987654
`
`ii
`
`ii
`
`

`

`3.10 TRANSMISSION AND DISTRIBUTION
`
`TRANSMISSION AND DISTRIBUTION
`
`145
`
`While the generation side of electric power systems usually receives the most
`attention, the shift toward utility restructuring, along with the emergence of dis—
`tributed generation systems, is causing renewed interest in the transmission and
`distribution (T&D) side of the business.
`_
`Figure 3.33 shows the relative capital expenditures on T&D over time com—
`__
`_' pared with generation by US. investor—owned utilities. The most striking feature
`of the graph is the extraordinary period of power plant construction that lasted
`Q"? '_ from the early 19705 through the mid-19805, driven largely by huge spending for
`nuclear power stations. Except for that anomalous period, T&D construction has
`generally cost utilities more than they have spent on generation. In the latter half
`:51 the 19905, T&D expenditures were roughly double that of generation, with
`
`
`most of that being spent on the distribution portion of T&D.
`»
`
`The utility grid system starts with transmission lines that carry large blocks
`power, at voltages ranging from 161 kV to 765 kV, over relatively long
`
`"diStances from central generating stations toward major load centers. Lower~
`
`ol'tage subtransmission lines may carry it to distribution substations located
`
`
`oser to the loads At substations, the voltage is lowered once again, to typi-
`
`ally 4 16 to 24.94 kV and sent out over distribution feeders to customers. An
`
`12 " mple of a simple distribution substation is diagrammed111 Fig. 3.34. Notice
`j__combination of switches, circuit breakers, and fuses that protect key corn—
`
`ents and which allow different segments of the system to be isolated for
`
`ten-ance or during emergency faults» (short circuits) that may occur in the
`
`
`
`
`
`—-—- Transmission and distribution - -- Generation
`
`
`
`
`
`
`30: 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 19961998
`Year
`
`
`Transmission and distribution (T&D) construction expenditures at US.
`ed;.'iiti_lities compared with generation. Except for the anomalous spurt in
`hs'truction during the 19705 and early 19805, T&D costs have generally
`ration From Lovins et al. (2002), using Edison Electric Institute data.
`
`
`
`

`

`146
`
`THE ELECTRIC POWER INDUSTRY
`
`I
`
`Substation
`Disconnect
`
`Substation
`Disconnect Relay
`Disconnect
`Transformer W
`Bus Breaker
`Q
`
`Overcurrent
`Feeder
`Distribution.
`_. Ag? W% Distribution
`
`Radial
`
`Sizdgls
`24.94 W
`
`‘
`
`Fuse 0
`__
`:L'.
`’-"
`
`
`
`_
`‘
`Subtransrnrssron
`System
`34.5 kV — 138 kV
`
`o
`i0
`I. Overcurrent
`‘
`Relay
`.
`
`F
`0
`
`_
`Lightning
`
`Arrestors FFeeder
`O
`Breakers
`
`Main Bus
`
`Voltage
`Regulators
`
`Figure 3.34 A simple distribution station. For simplication this is drawn as a one--line
`diagram, which means that a singie conductor on the diagram corresponds to the three '
`lines111 a three--phase system.
`-
`
`3.10.1 The National Transmission Grid
`
`
`
`The United States has close to 275,000 miles of transmission lines, most of which f;
`
`carry high-voltage, three-phase ac power. Investor owned utilities (IOUs) o'
`3-
`
`three—fourths of those lines (200,000 miles), with the remaining 75,000 miles :
`' owned by federal, public, and cooperative utilities. Independent power prodo
`ers do not own transmission lines so their ability to wheel power to cuSto:
`
`
`depends entirely on their ability to have access to that grid. As will be descn ed
`in the regulatory section of this chapter, Federal Energy Regulatory CorinhiSsi
`(FERC) Order 2000 is attempting to dramatically change the utility—cynicism
`
`of the grid as part of its efforts to promote a fully competitive wholesale-po
`market. Order 2000 encourages the establishment of independent regional'tr
`
`mission organizations (RTOs), which could shift transmission line ownerShi
`a handful of separate transmission companies (TRANSCOS), or it coul'l'
`
`continued utility ownership but with control turned over to independent,
`operators (ISOs).
`'
`1
`
`
`As shown1n Fig 3 35, the transmission networkin the United States,
`
`nized around three major power grids:
`the Eastern InterConnect,
`the W
`
`Interconnect, and the Texas Interconnect. Texas is unique in that its 'powe
`not cross state lines so it is not subject to control by the Federal Enef'
`
`ulatory Commission (FERC). Within each of these three interconnection
`utilities buy and sell power among themselves. There are very limite
`
`connections between the three major power grids. After a major blac
`the Northeastern United States in 1965, the North American Electric
`
`ity Council (NERC) was formed to help coordinate bulk power pol
`affect the reliability and adequacy of service within 10 designated. region
`U.S. grid
`-
`
`While almost all power in the United States is transmitted o
`
`phase ac transmission lines,
`there are circumstances in which. hlg
`
`
`
`

`

`TRANSMISSION AND DISTRIBUTION
`
`147
`
`
`
`Western
`interconnect
`
`
`
`
`Interconnect
`
`
`
`
`
`Texas Interconnect
`
`I‘li‘igiire 3.35 Transmission of US. electric power is divided into three quite separate
`" power grids which are further subdivided into 10 North American Electric Reliability
`
`
`
`__ ouncil Regions. ECAR, East Central Area Reliability Coordination Agreement; ERCOT,
`
`Electric Reliability Council of Texas; FRCC, Florida Reliability Coordinating Council;
`MAAC Mid—Atlantic Area Council; MAPP, Mid-Continent Area Power Pool; MAIN
`Mid—America Interconnected Network; NPCC, Northeast Power Coordinating Council;
`SERC Southeastern Electric Reliability Council; SPP, Southwest Power P001;WSCC,
`estemSystems Coordinating Council. (BIA 2001).
`
`
`
`
`HVDC) lines have certain benefits. They are especially useful for inter—
`
`fleeting the power grid in one part of the country to a grid in another
`"ine'e problems associated with exactly matching frequency, phase, and
`
`
`age:' are eliminated in dc. An example of such a system is the 600-
`GOOD-MW Pacific Intertie between the Pacific Northwest and South—
`,ahfprnia. Similar situations occur between countries, and indeed many
`
`HVDC links around the world are used to link the grid of one
`
`
`i '_'an0ther—examples include: Norway—Denmark, Finland—Sweden,
`
`Denmark, Canada—United States, Gennany—Czechoslovakia, Aus-
`gary, ArgentinaBrazil, France— England, and Mozambique— South
`
`Control and interfacing simplicity of do makes HVDC links particu-
`nited for connecting ac grids that operate with different frequencies,
`
`__e':_for example, in Japan, with its 50—Hz and 60-Hz regions.
`nits.require converters at both ends of the dc transmission line to
`
`Q‘dc'and then to invert do back to ac. The converters at each end
`Zelthfir as a rectifier or as an inverter, which allows power flow
`
`
`
`

`

`148
`
`THE ELECTRIC POWER wousmv
`
`AC Generators
`
`HVDC Link
`
`
` Breakers
`
`Loads
`
`W Rectifier
`AC System
`
` Transformer D~O
`
`Inverter
`
`‘--—-—-v—--J
`AC System
`
`Figure 3.36 A one—line diagram of a dc link between ac systems. The inverter and
`rectifier can switch roles to allow bidirectional pOWel' flow.
`
`in either direction. A simple one-line drawing of an HVDC link is shown in
`Fig. 3.36. HVDC lines offer the most economic form of transmission over very
`long distances—r—that is, distances beyond about 500 miles or so. For these longer
`distances, the extra costs of converters at each end can be more than offset by
`the reduction in transmission line and tower costs.
`'
`
`3.10.2 Transmission Lines
`
`:5 "i
`. f'i
`._ :;
`
`
`
`
`
`
`
`The physical characteristics of transmission lines depend very much on the volt— '
`ages that they carry. Cables carrying higher voltages. must be spaced further apart
`from each other and from the ground to prevent arcing from line to line, and-
`higher current levels require thicker conductors Table 3.5 lists the most common L
`
`voltages in use in the United States along with their usual designation as beingE
`transmission, subtransmission, distribution, or utilization voltages.
`.
`"
`
`Figure 3. 37 shows examples of towers used for various representative trans
`mission and subtransmission voltages Notice that the 500—kV tower has the ’
`
`suspended connections for the three—phase current, but it also shows a fourth
`connection, namely, a ground Wire above the entire structure. This ground} Wir'
`not only serves as a return path1n case the phases are not balanced, butalso
`provides a certain amount of lightning protection.
`"
`
`TABLE 3.5 Nominal Standard T&D System Voltages
`
`Transmission (kV)
`
`Subtransmission (kV) Distribution (kV)
`
`i
`
`765
`500
`345
`230
`161
`
`g
`
`138
`115‘
`69
`46
`34.5
`
`.
`
`‘
`
`24.94
`22.86
`13.8
`13.2 7
`12.47 '
`
`8 32
`
`4.16
`
`'
`
`
`
`
`Utilizationgflg’l
`
`'
`
`600;: --
`480;;
`24o.;.-_g
`208.3,
`1203*
`
`

`

`»M.W
`
`i CHAPTER 6
`
` WIND POWER SYSTEMS
`
`
`
`6:1. HISTORICAL DEVELOPMENT OF WIND POWER
`
`
`
`
`
`Wind has been utilized as a source Of power for thousands of years for such
`asks as propelling sailing ships, grinding grain, pumping water, and powering
`
`ctOry machinery. The world’s first wind turbine used to generate electric—
`
`
`ityfwas built by 21 Dane, Poul
`la Cour, in 1891. It is especially interesting
`---_h'ote that La Cour used the electricity generated by his turbines to elec—
`
`
`ily'Ze water, producing hydrogen for gas lights in' the local schoolhouse. In
`
`at'regard we could say that hewas 100 years ahead of his time since the
`
`sionythat many have for the twenty--first century includes photovoltaic and
`
`_d-ipower systems making hydrogen by electrolysis to generate electric power
`fuel Cells.
`
`_ the United States the first wind—electric systems were built in the late
`0s;b_y the 19303 and 19403, hundreds Of thousands of small—capacity, wind—
`
`trie' systems were in use in rural areas not yet served by the electricity
`
`Iii-1941 one of the largest wind-powered systems ever built went into
`
`' tion at Grandpa’s Knob in Vermont. Designed to produce 1250 kW from
`
`
`-ft_~d1ameter two-bladed prop,
`the unit had withstood winds as high as
`
`___Ies per hour before it catastrophically failed in 1945 in a modest 25:
`nii'_.'(one ofits 8-ton blades broke loose and was hurled 750 feet away).
`
`
`
`
`
`
`e and Efficient Electric Power Systems. By Gilbert M. Masters
`.47128060-7 © 2004 John Wiley & Sons, Inc
`
`307
`
`

`

`308
`
`WIND POWER SYSTEMS
`
`Subsequent interest in wind systems declined as the utility grid expanded and
`became more reliable and electricity prices declined. The oil shocks of the 1970s,
`which heightened awareness of our energy problems, coupled with substantial
`financial and regulatory incentives for alternative energy systems, stimulated a
`renewal of interest in windpower. Within a decade or so, dozens of manufac~
`turers installed thousands of new wind turbines (mostly in California). While
`many of those machines performed below expectations, the tax credits and other
`incentives deserve credit for shortening the time required to sort out the best
`technologies. The wind boom in California was short-lived, and when the tax
`credits were terminated in the mid-19803, installation of new machines in the
`
`United States stopped almost completely for a decade. Since most of the world’s
`wind—power sales, up until about 1985, were in the United States,
`this sud- -_
`den drop in the market practically wiped out the industry worldwide until the
`early 19903.
`Meanwhiley, wind turbine technology development continued—especially in
`Denmark, Germany, and Spain—and those countries were ready when sales 3;:
`began to boom in the mid—1990s. As shown in Fig. 6.1,
`the global installed-2.;
`capacity of wind turbines has been growing at over 25% per year.
`1.2.
`Globally, the countries with the most installed wind capacity are shown in
`Fig. 6.2. As of 2003, the world leader is Germany, followed by Spain, the United:
`States, Denmark, and India. In the United States, California continues to have the
`most installed capacity, but as shown in Fig. 6.3, Texas is rapidly closing the gap; -2
`Large numbers of turbines have been installed along the Columbia River Gorge itijij
`the Pacific Northwest, and the windy Great Plains states are experiencing major”?
`growth as well.
`'
`-
`'
`
`I.
`
`I
` 3 Net Additions
`
`
`.{
`
`Installed Capacity
`
`40,000
`
`35,000
`
`30,000
`
`25,000
`
`20,000
`
`
`
`Capacity(MW)
`
`
`
`i5.0001
`
`10.000
`
`5000
`
`Figure 6.1 Worldwide installed wind-power capacity and net annual additidnstb C
`ity have grown by over 25% per year since the mid-19905. Data from AWEA-
`
`
`
`

`

`TYPES OF WIND TURBINES
`
`309
`
`4,685 Netherlands, 688
`
`'
`
`Italy, 785
`
`USA
`
`India
`
`1,702
`
`Denmar
`
`
`2,880
`
`
`
`Germany
`12,001
`
`Figure 6.2 Tetal installed capacity in 2002, by country. AWEA data.
`
`
`
`2000
`
`1500
`
`1 000
`
`
`
`
`
`
`
`500
`
`.9El.—
`
`:2
`"e
`0
`
`Texas
`
`lowa
`
`Minnesota
`
`Washington
`
`Oregon.
`
`Wyoming
`
`Kansas
`
`'- Figure 6.3
`
`Installed wind capacity in the United States in 1999 and 2002.
`
`
`
`SPES 0F WIND TURBINES
`
`
`Sally Wind turbines were used to grind grain into flour, hence the name
`
`mill.” Strictly Speaking, therefore, calling a machine that pumps water or
`
`6S electricity a windmill is somewhat of a misnomer. Instead, people are
`
`
`

`

`310
`
`WIND POWER SYSTEMS
`
`using more accurate, but generally clumsier, terminology: “Wind-driven gener-
`1’
`fit
`ator, wind generator,” “wind turbine,” “wind-turbine generator” (WTG), and
`“wind energy conversion system” (WECS) all are in use. For our purposes,
`“wind turbine” will suffice even though often we will be talking about system
`components (e. g., towers, generators, etc.) that clearly are not part of 'a “turbine.”
`' One way to classify wind turbines is in terms of the axis around Which the
`turbine blades rotate. Most are horizontal axis wind turbines (HAWT), but there
`are some with blades that spin around a vertical axis (VAWT). Examples of the
`two types are shown in Fig. 6.4.
`The only vertical axis machine that has had any commercial success is the
`Darrieus rotor, named after- its inventor the French engineer G. M. Darneus,
`who first developed the turbines in the 19205. The shape of the blades is that
`which would result from holding a rope at both ends and spinning it around a
`vertical axis, giving it a look that is not unlike a giant eggbeater. Considerable
`development of these turbines, including a SOD-kW, 34—m diameter machine, was
`undertaken in the 1980s by Sandia National Laboratories in the United States.
`An American company, FloWind, manufactured and installed a number of these
`wind turbines before leaving the business in 1997.
`The principal advantage of vertical axis machines, such as the Darrieus rotor,
`is that they don’t need any kind of yaw control to keep them facing into the
`wind. A second advantage is that the heavy machinery contained in the nacelle
`(the housing around the generator, gear box, and other mechanical components)
`can be located down on the ground, where it can be serviced easily. Since the
`heavy equipment is not perched on top of a tower, the tower itself need not
`be structurally as strong as that for a HAWT. The tower can be lightened even
`further when guy wires are used, which is fine for towers located on land but not
`for offshore installations. The blades on a Darrieus rotor, as they spin around, are
`almost always in pure tension, which means that they can be relatively lightweight
`
`Gear
`w .. box Generator-fl
`
`
`
`._—__..'..
`
`,
`_
`M '
`Wind
`
`Nacelle
`/ Tower
`
`Rotor
`blades
`
`--‘
`,
`.=:-:<~...:.
`Upwin
`HAWT
`
`(a)
`
`M _
`
`Wind
`
`Downwind
`HAWT
`
`(b)
`
`Guy wires
`/
`
`
`Wind
`
`Rotor blades
`‘/
`
`Generator,
`Gear Box
`
`Wind
`
`
`I
`
`‘
`Darrieus
`VAWT
`
`(C)
`
`
`
`Figure 6.4 Horizontal axis wind turbines (HAWT) are either upwind machines (a) 01"“
`downwind machines (b). Vertical axis wind turbines (VAWT) accept the wind from BUY
`direction (c).
`.
`
`
`
`

`

`336
`
`WIND POWER SYSTEMS
`
`
`
`
`
`Powerdelivered(kW)
`
`400
`
`03OO
`
`100
`
`200
`
`Wind speed (m/s)
`
`Figure 6.20 Example of the impact that a three-step rotational speed adjustment has
`on delivered power. For winds below 7.5 111/3, 20 rpm is best; between 7.5 and 11 m/s,
`30 rpm is best; and above 11 m/s, 40 rpm is best.
`
`
`
`
`
`
`
`
`
`
`
`shows the impact of varying rotor speed from 20 to 30 to 40 rpm for a 30-m
`rotor with efficiency given in Fig. 6.19, along with an assumed gear and generator
`efficiency of 70%.
`7 While blade efficiency benefits from adjustments in speed as illustrated in
`Figs. 6.19- and 6.20, the generator may need to spin at a fixed rate in order to
`deliver current and voltage in phase with the grid that it is feeding. So, for
`grid—connected turbines, the challenge is to design machines that can somehow
`accommodate variable rotor speed and somewhat fixed generator speed—or at
`least attempt to do so. If the wind turbine is not grid-connected, the generator
`electrical output can be allowed to vary in frequency (usually it is converted to 1
`dc), so this dilemma isn’t a problem.
`'
`
`
`
`
`
`6.7.2 Pole-Changing Induction Generators
`
`Induction generators spin at a frequency that is largely controlled by the number
`of poles. A two-pole, 60-Hz generator rotates at very close to 3600 rpm; with
`four poles it rotates at close to 1800 rpm; and so on. If we could change the
`number of poles, we could allow the wind turbine to have several operating
`speeds, approximating the performance shown in Figs. 6.19 and 6.20. A key to
`this approach is that as far as the rotor is concerned, the number of. poles in
`
`the stator of an induction generator is irrelevant. That is, the stator can have
`external connections that switch the number of poles from one value to anotheF
`
`without needing any change in the rotor. This approach is common in householt
`appliance motors such as those used in washing machines and exhaust fans
`give two— or three—speed operation.
`
`
`
`
`
`

`

`(cid:14)(cid:12)(cid:5)(cid:5)(cid:4)(cid:19)(cid:3)(cid:11)(cid:10)(cid:15)(cid:13)(cid:11)(cid:8)(cid:19)(cid:6)(cid:11)(cid:13)(cid:19)(cid:9)(cid:2)(cid:7)(cid:9)(cid:16)(cid:9)(cid:19)(cid:12)(cid:11)(cid:17)(cid:5)(cid:13)(cid:19)
`SPEED CONTROL FOR MAXIMUM POWER
`
`(cid:1)(cid:1)(cid:18)(cid:19)
`333
`
`(cid:7)(cid:2)(cid:9)(cid:2)(cid:4)(cid:35) (cid:14)(cid:32)(cid:24)(cid:31)(cid:22)(cid:28)(cid:23)(cid:21)(cid:35)(cid:12)(cid:21)(cid:17)(cid:29)(cid:18)(cid:27)(cid:33)(cid:21)(cid:30)(cid:35)
`6.7.3 Multiple Gearboxes
`(cid:33)(cid:95)(cid:84)(cid:51)(cid:134) (cid:123)(cid:74)(cid:90)(cid:45)(cid:134) (cid:113)(cid:120)(cid:101)(cid:39)(cid:74)(cid:90)(cid:51)(cid:112)(cid:134) (cid:68)(cid:38)(cid:121)(cid:51)(cid:134) (cid:113)(cid:123)(cid:95)(cid:134) (cid:67)(cid:51)(cid:38)(cid:102)(cid:39)(cid:95)(cid:126)(cid:51)(cid:112)(cid:134) (cid:123)(cid:74)(cid:113)(cid:68)(cid:134) (cid:112)(cid:51)(cid:97)(cid:38)(cid:102)(cid:38)(cid:113)(cid:51)(cid:134) (cid:67)(cid:51)(cid:90)(cid:51)(cid:102)(cid:38)(cid:113)(cid:95)(cid:102)(cid:112)(cid:134) (cid:38)(cid:113)(cid:113)(cid:38)(cid:40)(cid:68)(cid:51)(cid:45)(cid:134) (cid:113)(cid:95)(cid:134)
`Some wind turbines have two gearboxes with separate generators attached to
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`each, giving a low—wind-speed gear ratio and generator plus a high-wind-speed
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`gear ratio and generator.
`
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`6.7.4 Variable-Sliplnduction Generators
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`A normal induction generator maintains its speed within about 1% of the syn--
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`chronous speed. As it turns out, the slip in such generators is a function of the
`(cid:45)(cid:54)(cid:134)(cid:102)(cid:51)(cid:112)(cid:74)(cid:112)(cid:113)(cid:38)(cid:90)(cid:40)(cid:51)(cid:134)(cid:74)(cid:90)(cid:134)(cid:113)(cid:68)(cid:51)(cid:134)(cid:102)(cid:95)(cid:113)(cid:95)(cid:102)(cid:134)(cid:40)(cid:95)(cid:90)(cid:45)(cid:118)(cid:40)(cid:113)(cid:95)(cid:102)(cid:112)(cid:11)(cid:134)(cid:23)(cid:127)(cid:134)(cid:97)(cid:118)(cid:102)(cid:97)(cid:95)(cid:112)(cid:51)(cid:83)(cid:127)(cid:134)(cid:38)(cid:45)(cid:45)(cid:74)(cid:90)(cid:67)(cid:134)(cid:121)(cid:38)(cid:102)(cid:74)(cid:38)(cid:39)(cid:83)(cid:51)(cid:134)(cid:102)(cid:51)(cid:112)(cid:74)(cid:112)(cid:113)(cid:38)(cid:90)(cid:40)(cid:51)(cid:134)(cid:113)(cid:95)(cid:134)
`[5}: dc resistance in the rotor conductors. By purposely adding variable resistance to
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`53;;- the rotor, the amount of slip can range up to around 10% or so, which .would
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`mean, for example, that a four-pole, 1800-rpm machine could operate anywhere
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`3:2-from about 1800 to 2000 rpm. One way to provide this capability is to have
`(cid:38)(cid:45)(cid:81)(cid:118)(cid:112)(cid:113)(cid:38)(cid:39)(cid:83)(cid:51)(cid:134) (cid:102)(cid:51)(cid:112)(cid:74)(cid:112)(cid:113)(cid:95)(cid:102)(cid:112)(cid:134) (cid:51)(cid:126)(cid:113)(cid:51)(cid:110)(cid:38)(cid:83)(cid:134) (cid:113)(cid:95)(cid:134) (cid:113)(cid:68)(cid:52)(cid:134) (cid:67)(cid:51)(cid:90)(cid:51)(cid:102)(cid:38)(cid:113)(cid:95)(cid:102)(cid:3)(cid:134) (cid:39)(cid:118)(cid:113)(cid:134) (cid:113)(cid:68)(cid:51)(cid:134) (cid:113)(cid:102)(cid:38)(cid:45)(cid:51)(cid:6)(cid:95)(cid:61)(cid:134)(cid:74)(cid:112)(cid:134) (cid:113)(cid:68)(cid:38)(cid:113)(cid:134) (cid:90)(cid:95)(cid:123)(cid:134) (cid:38)(cid:90)(cid:134)
`adjustable resistors external to the generator, but the trade-off is that now an
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`5'..E33;electrical connection is needed between the rotor and resistors. That can mean
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`"f2"; _..abandoning the elegant cage rotor concept and instead using a wound rotor with
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`" lip rings and brushes similar to what a synchronous generator has. And that
`
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`)means more maintenance will be required.
`
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`I Another way to provide variable resistance for the rotor is to physically mount
`I'
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`
`.. he resistors and the electronics that are needed to control them on the rotor itself.
`(cid:24)(cid:118)(cid:113)(cid:134)(cid:113)(cid:68)(cid:51)(cid:90)(cid:134)(cid:127)(cid:95)(cid:118)(cid:134)(cid:90)(cid:51)(cid:51)(cid:45)(cid:134)(cid:112)(cid:95)(cid:84)(cid:51)(cid:134)(cid:123)(cid:38)(cid:127)(cid:134)(cid:113)(cid:95)(cid:134)(cid:112)(cid:51)(cid:90)(cid:45)(cid:134)(cid:112)(cid:74)(cid:67)(cid:90)(cid:38)(cid:83)(cid:112)(cid:134)(cid:113)(cid:95)(cid:134)(cid:113)(cid:68)(cid:51)(cid:134)(cid:102)(cid:95)(cid:113)(cid:95)(cid:102)(cid:134)(cid:113)(cid:51)(cid:83)(cid:83)(cid:74)(cid:90)(cid:67)(cid:134)(cid:74)(cid:113)(cid:134)(cid:68)(cid:9